This invention relates to light modulators and, more particularly, to high-speed light modulators whose operation is based on varying the electro-optic properties of liquid crystal materials.
Liquid crystal light modulators are useful in a variety of applications of practical importance. They may, for example, be employed in a data communication system to modulate a serial stream of optical pulses. Or such modulators may be arranged in a two-dimensional array to form a flat panel display.
Many available liquid crystal light modulators exhibit relatively slow response times. In practice, this property limits the suitability of such modulators for use in high-bit-rate communication systems and large-area high-definition displays.
One of the factors contributing to the slow response time of liquid crystal light modulators is that most of them as heretofore proposed respond to a quadratic component of the electric field E, that is, they are E.sup.2 dependent. Once the field (of either polarity) is removed, the liquid crystal relaxes to its zero-field state. These relaxation times are typically long and not electrically controllable.
There are two known references which disclose a linear or polarity-sensitive electric-field effect in a liquid crystal material. G. E. A. Durand et al in U.S. Pat. No. 4,564,266 disclose an electro-optic device in which different alignment layers are utilized to impose parallel and perpendicular orientations on the molecules at opposed surfaces of a liquid crystal layer. In operation, the Durand et al device requires electrodes that are transversely arranged with respect to the direction of light propagation through the device. For a large-area display that includes a layer of liquid crystal material that is only, for example, about 10 micrometers thick, such an electrode array is generally not feasible.
The second known reference that discloses a linear electric-field effect in a liquid crystal material is an article by N. V. Madhusudana et al entitled "Linear Flexo-Electro-Optic Effect In A Hybrid Aligned Nematic Liquid Crystal Cell", J. Physique Lett., Vol. 46, 1985, pages L195-L200. As in the aforecited Durand et al patent, different alignment layers are utilized to impose parallel and perpendicular orientations on the molecules at opposed surfaces of a liquid crystal layer. Due to a bulk effect in the material, the molecular orientation can be altered by applying an electric field perpendicular to the liquid crystal surfaces. No particular device applications for such an arrangement are suggested in the article.
Moreover, in both of the aforecited references, linear operation is limited to low voltages to avoid dielectric contributions that are polarity-insensitive. Accordingly, the speed of operation of the devices described therein is relatively slow in the linear region.